In diesel engine motor vehicles, a glow plug is often used to ignite the fuel on those occasions when the fuel temperature is not high enough to allow combustion. Glow plugs designed for fast response may be protected from overheating by the use of electrothermally actuated switches or bimetal contacts which are opened and closed in response to a flow of current heating the switch. Such a system automatically compensates for the effects of voltage and ambient temperature on the glow plug's temperature.
A typical glow plug controller may have three heater operated bimetal contacts used in series to control a relay coil having heavy duty contactors. The contactors are connected in series between the glow plugs and the source of electrical power.
The bimetal contacts controlling the relay coil, are required to make and break electrical current paths. As such, considerable electrical arcing may occur across the contacts resulting in contact failure. In a typical controller, there are three such bimetal contacts in series. The first contact is a circuit breaker contact and generally does not operate except in response to malfunction. The second contact is a thermal oscillator and operates to modulate the temperature of the glow plugs. The thermal oscillator contacts energize and deenergize a relay coil and as such make and break a relatively heavy current load. The third contact, the afterglow contact, controls the time that the glow plug controller operates. It is normally closed and is opened from electrical current supplied to its heater from the alternator after the engine is started and running. Because the contact timings are asynchronous, the afterglow contacts may break the relay coil current.
The opening of the bimetal contacts when current is being supplied to the relay results in an arc across the contacts and eventual destruction of the contacts. When the contacts are destroyed, the vehicle may not start and there are times when such non-starting may result in a dangerous situation.